Rain
A familiar example of the origin and nature of Rain is seen in the formation of a miniature shower, when steam escapes from a steam-engine into the air. When water is heated, it becomes an invisible vapour; and this rises, and mixes with the air, if the air is warm. When the warm air, full of water in this state, mixes with cold air, or touches any cold object, the vapour returns to the state of water; but, for a time, it remains in the form of very small particles, like fine dust--so light, that they can still float in the air. In this state, they form clouds when high above the earth, or mist when lying close to the earth. Among mountains, clouds and mists are often seen to be directly continuous. But mists often cling to the mountain-tops while the sky is nearly cloudless. This is owing to the rocks and soil being colder than the air, and causing the vapour to become visible as mist, by cooling the air near them below the temperature at which the water can remain vapour. As the wind carries the mist from the mountain into the warmer air around, it resumes the form of vapour, and becomes invisible; but new mist is formed, and supplies the place of that carried away, so that the mist seems to remain unchanged on the peaks for hours. Clouds form and disappear in the sky according as the air filled with vapour meets colder or warmer winds. When the minute drops of water that form clouds are very crowded, they join together, and form drops, too large to be supported in the air. These then fall as Rain. Sometimes, a cloud will be seen raining high up in the sky, though Rain does not reach the ground. This happens when the Rain has to fall through a layer of warm, dry air, in which it is all turned into vapour again. Light Rain sometimes falls out of a sky cloudless or nearly so; but this is a rare occurrence. The greater amount of rainfall during the night is due to the cooling of the air when the sun's heat is withdrawn. The vapour forms drops of water, and falls as Rain. The sources of the vapour in the air are various. Much of it is given off by growing plants, and a little by animals. More is taken up by warm, dry winds blowing over the surface of the land, with its marshes, rivers, and lakes; but most of all is absorbed by the winds in passing over the oceans, such as the Atlantic Ocean. Such winds are saturated, or nearly so; that is, they carry away as much vapour as they can dissolve at the temperature they had while passing over the water. If this is higher than that of the land at which they first arrive, they will give up, as Rain, the vapour that no longer can be dissolved when the temperature falls. Hence, countries near oceans receive more Rain than those at a distance from them in the middle of continents. Mountainous countries near the sea are especially rainy, since air becomes colder the higher it rises along the slopes, and a very great part of the vapour is thus lost. For the reasons just stated, as well as from local peculiarities of situation, the frequency and amount of the rainfall varies widely in different localities, and at different seasons. The total amount of Rain that falls in any given time is measured by means of instruments (see Rain-gauge). Careful observations have been carried on in many countries, and in many localities, for a considerable number of years, as to the total amount of Rain that falls during the year, as well as during the various periods of the year, in each locality. The amount is expressed in the number of inches in depth that the Rain would reach in any given time, if it could all be confined to the exact area on which it has fallen. It has been found that, in some countries in the tropics, e.g., in Upper Egypt and the Sahara Desert, and on part of the coast of Peru, little Rain, if any, falls. On the other hand, the heaviest recorded rainfall occurs among mountain ranges in the neighbourhood of tropical oceans. Thus, in the Himalaya Mountains, about 100 miles from Calcutta, a rainfall of about 524in., or about 44ft., has been observed within a year. In the British Islands, the rainfall is considerably greater on the west side than on the east, the winds from the Atlantic Ocean losing much of their vapour among the mountains. The heaviest rainfall in Britain is recorded from the Cumberland mountains, near Keswick, where over 150in., or 12 1/2ft., of Rain has been collected in a year. Along our western coasts, the average annual rainfall varies from about 36in. to 66in., and on the eastern side of our islands, from about 20in. to 30in. But the actual rainfall does not bear any definite relation to the number of rainy days in a year, for often the heaviest fall is met with in districts where it is almost limited to certain seasons. When much Rain falls in a limited time, the greater part of it necessarily flows off the surface of the ground, and it is apt to carry away the fertile soil, and to cause disastrous floods. Where, on the other hand, the period during which it falls is more prolonged, the Rain sinks into the soil, and supplies the underground reservoirs of springs.
In passing through the atmosphere, the rain becomes of the same temperature, and, as this in summer is almost always higher than that of the soil, the latter becomes warmed, and the plants in it are stimulated to more active growth. Moreover, the rain-water has oxygen, and, it may be, also minute traces of ammonia and of nitric acid, obtained from the atmosphere, dissolved freely in it, and these also are beneficial, as food, to the roots of plants. It is unnecessary to dilate on the importance of a good supply of water in the soil, insured to plants by regular Rains.
Rain-water is very generally preferred for watering plants in pots, and it has the great advantage over that drawn from springs, or from underground pipes, of usually being at the temperature of the air, and of containing in it the oxygen, and other substances, mentioned above. Its use is of course similar, in its effects, to the watering of plants by Rain in the open air and, when rain-water can be had, the employment of it is therefore to be recommended.